The invention herein described was made in the course of or under a contract with the Department of the Navy.
1. Field of the Invention
This invention relates to gas turbine engines and more particularly to engines having nozzle guide vanes which are both rotatable and coolable.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, pressurized air and fuel are burned in a combustion chamber to add thermal energy to the medium gases flowing therethrough. The effluent from the chamber comprises high temperature gases which are flowed downstream in an annular flow path through the turbine section of the engine. Nozzle guide vanes at the inlet to the turbine direct the medium gases onto a multiplicity of blades which extend radially outward from the engine rotor. The nozzle guide vanes are particularly susceptible to thermal damage and are commonly cooled to control the temperature of the material comprising the vanes. Cooling air from the engine compressor is bled through suitable conduit means to an annular chamber which is located radially outward of the working medium flow path and thence to the vanes. The nozzle guide vanes in conventional constructions have platforms which separate the cooling air in the chamber from the working medium gases in the flow path.
Recent efforts to improve the performance of gas turbine engines have led to the development of turbines having variable geometry nozzles. In a typical construction such as that shown in U.S. Pat. No. 3,224,194 to DeFeo et al entitled "Gas Turbine Engine", the area of the turbine nozzle is varied with the engine power level to optimize the flow characteristics of the working medium gases in the region. In DeFeo a plurality of rotatable vanes are positioned circumferentially about the medium flow path to form the turbine nozzle. The ends of each vane are affixed to their respective supporting structure by ball and socket type connectors. The connectors accommodate minor variations in the angle of the vane radial axis which are caused by differential axial expansion between the supporting structures.
Some newly developed engines have incorporated rotatable vanes which are cantilevered from the outer case structure to eliminate the deleterious effects on the vanes of thermal expansion between the cases. Typically, as is shown in U.S. Pat. No. 3,542,484 to Mason entitled "Variable Vanes" and in U.S. Pat. No. 3,652,177 to Loebel entitled "Installation for the Support of Pivotal Guide Blades", the vanes are mounted from the outer case and extend radially inward toward but independently of the inner case. In both constructions the axial gas pressure load on each vane is transmitted to the outer case through a cylindrical bushing which surrounds the stem of the vane. The radial gas pressure load on each vane is transmitted in Mason through a bearing ring to the outer case and in Loebel through the cylindrical bushing to the outer case. An inherent problem with cantilevered vane constructions is the control of medium gas leakage between the tip of each vane and the surrounding shroud at the inner case. The shroud is supported by the inner engine case and is displaced radially according to the thermal response characteristics of the inner case. On the other hand, the vanes are supported by the outer case and are displaced radially according to the thermal response characteristics of the outer case. In most constructions a substantial initial clearance is provided to prevent binding between the vanes and the inner shroud under transient conditions with the result that leakage is excessive during nearly all periods of operation. The leakage problem is particularly acute in high temperature engines where the relative radial displacement due to thermal expansion is excessive between the inner shroud and the vane tips.
In high temperature engines cooling air from the compressor is commonly flowable between the outer engine case and an outer shroud surrounding the flow path. Each rotatable vane has a cylindrical platform which is integral with the outer shroud. The airfoil section of each vane extends beyond its respective platform and overhangs a portion of the outer shroud. Sufficient, clearance between the overhung portion of each vane and the shroud must be provided to insure rotatability of the vane without binding against the shroud. An excessive clearance, however, deleteriously effects the aerodynamic performance of the turbine by allowing the medium gases to leak past the overhung region without being fully redirected by the airfoil.
Continuing efforts are underway to provide turbine apparatus which in combination with rotatable vanes allows variations in nozzle area with minimized leakage of the working medium gases.